ABSTRACT South Africa is home to the world's largest antiretroviral therapy (ART) program, but sustaining high retention along the HIV care continuum has proven challenging in the country and throughout the wider region. Population mobility is common in South Africa, but important research gaps exist describing this mobility and its impact on engagement in HIV care. Postpartum women and their infants in South Africa are known to be at high risk of dropping out of HIV care after delivery and are frequently mobile. We recently developed a beta version of a smartphone application (app) ? CareConekta ? that detects a user's smartphone location to allow for prospective characterization of mobility. Through this three-year award, we propose to adapt and test CareConekta to conduct essential formative work on mobility and evaluate an intervention ? the CareConekta app plus text notifications and optional staff phone calls and/or WhatsApp messages ? to facilitate engagement in HIV care during times of mobility. During the three-year project period, we propose these aims: 1) adapt CareConekta to use appropriate smartphone technology for the local population; 2) characterize mobility among South African women during the peripartum period and its impact on engagement in HIV care; and 3) evaluate the acceptability, feasibility, and initial efficacy of using CareConekta as an intervention to improve engagement in HIV care. We will work with a South African technology company with a strong connection to the South African Department of Health to adapt the app. We will observe mobility within an observational cohort and will then assess the impact of the intervention among participants who travel during the study period. This work is critical to adapting an existing app for widespread use, providing critical information about mobility during the peripartum period and the impact on engagement in HIV care, and piloting an intervention to improve engagement. This study will lay the necessary groundwork for a larger efficacy trial of the intervention within different geographic settings.